Technical Field
This disclosure is directed to electronic circuits, and more particularly, to voltage-controlled oscillators.
Description of the Related Art
Phase-locked loops (PLLs) are commonly used in electronic circuit and integrated circuits (ICs) to generate and control the frequency of clock signals. A typical PLL includes a voltage-controlled oscillator (VCO) that generates the output clock signal based on a reference signal and a divisor ratio used in generating a feedback signal.
There are two types of VCOs most commonly used in PLLs. A first type is a ring oscillator type, which includes an odd number of inverters coupled with a feedback loop to maintain oscillation. A second type is implemented using an inductive-capacitive (LC) tank circuit. An LC tank circuit includes a capacitance and an inductance coupled in a parallel tank topology. The output signal may be taken from either of the junctions between the capacitor and the inductor. Tuning of the LC tank circuit may be accomplished by providing a tuning voltage to a variable capacitor of the capacitance. This may change the overall capacitance in the LC tank circuit, and thus change the frequency of oscillation of the circuit.